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Discussion Starter · #1 ·
Yeah. I know. Go to the bike shop for fit help. I am. Can you give me some advice before I make the over 1 hour trek to the LBS? My road bikes have around a 55cm top tube. And I'm using about a 12cm stem (old quill-style with no upward angle). I want to dive right into a higher-end mtb. So, I'd like to know how mtb top tubes compare to road top tubes.

Should I get a mtb with a top tube longer than my road bike top tube? By how much? And what size stem should I start out with to experiment?
 

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Discussion Starter · #3 ·
johnnychimpo said:
MTB dont work that way. You need a medium frame IMO. Trek would be around 17.5/19.5

Every mfg is different. Try checking out www.mtbr.com and asking your question there. You might find more people willing to help out.



Thanks. I'll start out with the 17.5" frames to see how they feel and go from there. What size mtb stem should I be looking at? I'm a 12cm stem on the road.
 

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Discussion Starter · #6 ·
I have my bike fit dialed in with my road bike. There ought to be some sort of easy way to just convert your seat tube, top tube, stem length preferences to a mtb.

road frame: medium
road top tube: 55cm
road stem: 12cm

mtb frame: medium
mtb top tube: around 22-23"
mtb stem: ???????

What size stem should I get? I have no idea. The fit calculator didn't do it for me.
 

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barbedwire said:
I have my bike fit dialed in with my road bike. There ought to be some sort of easy way to just convert your seat tube, top tube, stem length preferences to a mtb.

road frame: medium
road top tube: 55cm
road stem: 12cm

mtb frame: medium
mtb top tube: around 22-23"
mtb stem: ???????

What size stem should I get? I have no idea. The fit calculator didn't do it for me.
I would tend to go for a similar saddle to bar length however with a MTB a longer top tube and shorter stem would be a better combo.
A MTB stem of 120mm is considered long, 90-100mm would be better for handling/steering

As an example:
BMC SLT01 roady has a TT 585mm & 110mm stem & a setback post of 25mm = 720mm

Ibis Mojo MTB Dually has a TT 620mm & 100mm Stem & zero setback post = 720mm

GT Carbon Zaskar Hard Tail TT 617 & 100mm stem & setback post 20mm = 737mm

Notwithstanding that the MTB's have flat/straight bars and when on the roady I spend most of the time 'on the hoods' which makes the reach a little more 'streched' and the GT is also a little longer as its a XC racer and therefore also 'streched'

With a MTB it all comes down to the intended use/type ie: Trail Riding or racing? & Hard tail or full suspension (Dually)
 

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I think the idea that there should be a basic conversion factor is only valid if the type of riding done is going to be the same. It's a lot easier to spend more time moving around on an MTB on the trail, your posture is going to be different, the braking force is going to be very different. It's a whole other kind of thing.

Slower riding on trails that aren't clean is easier (I find) with shorter stems... 50-75mm. I find that with such lengths, steering is more intuitive and reactionary. Look that way, the bike will go there, kind of thing. On the other hand if you're bombing clean fire roads at a reasonable rate of speed, a longer stem will be more to your liking, as it stabilizes the steering a bit.

Road bikes look like road bikes. I'm sure there are a few differences in genetics, but basically, they're very well dialed in for the road. MTB can look like a lightweight XC racer, a beefy trials bike, a singlespeed huckster, a full suspension XC all the way to a serious DH kind of thing, or an ice bike, or more. Hard tail, soft tail, full suspension, shock, no shock, how much travel... 1.5 inch tires? 2.5"? Dialing in will not just be about fitting the rider, it will be about fitting the terrain.

If you're really looking for a clean conversion, maybe you should look into cyclocross instead of MTB. Basically, a road bike built for more abuse. (I'm sure there's more to it than my simplification, but you get the idea.)

If they still do things the same way, the Trek/Fisher lines are pretty close to identical, except that the Fisher has a longer top tube and a shorter stem. The fit for the rider is essentially the same, but, the handling is different. Try both a fisher and a Trek, and see which you like.

Other than that, yep. New sport, new equipment different deal. Try the bikes, find one you like. Make sure you have plenty of clearance between your top tube and your junk. And be aware that the bike is going to feel a lot different... MTBs these days seem to have a much higher center of gravity for better ground clearance.
 

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You're not going to find the same riding position on a mountain bike as you have on a road bike. Having said that, keep in mind that a road bike drop bars put the controls of the bike forward and down, necessitating a shorter top tube compared to a mountain bike with a flat bar. For example, my road bikes have top tubes of about 525mm with about a 100mm stem but my full-suspension Cannondale Prophet has a top tube that is more like 550mm with a 90mm stem. My riding position is more upright on this bike, which means that saddle comfort is much more important on that bike than on my road bikes because, though I'm sitting down less while riding offroad, I do put more pressure on my butt than when I ride a road bike when I do sit down. Full suspension allows me to stay in the saddle much more than you would think.

I think the bottom line is that you try the bike you like in a few different sizes and see which one feels right to you. While you can adjust the fit after the purchase, it is easier to get it almost right from the get-go then adjust accordingly than to buy something that is completely wrong, size-wise, and try to make it work. I won't make that mistake again.
 
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